1. Field of the Invention
Bis(ethynylphenyl) compounds, also known as ethynylated bis-aryl derivatives, are important intermediates in the synthesis of thermally stable resins for use in high-temperature structural composites and high-char yielding structure matrices, such as carbon-carbon composites. These materials are used in the fabrication of reentry missile nose cones, leading edges, rocket nozzles, and other structural applications requiring high structural strength and high thermal stability. The invention disclosed below describes a simplified process for preparing these intermediates in high yields that may readily be adapted to large-scale synthesis operations.
2. Description of the Prior Art
Classical methods for the synthesis of terminal aryl acetylene in general involve manipulation of preformed two-carbon side chains and include methods such as the Vilsmeier method, the halogenation dehydrohalogenation sequence of vinyl aromatics and aromatic ketones and the dehydrohalogenation of .beta.,.beta.-dihalo olefins. Methods that deviate from the classical approach have utilized the decomposition of preconstructed heterocycles. More recently, acetylenic substituents have been introduced onto aromatic nuclei by the Stephens-Castro coupling reaction. Since this reaction requires a stoichiometric quantity of an acetylenic copper reagent, prior preparation of such a reagent is needed, which consumes time, materials and energy. Also, the Stephens-Castro reaction requires the use of end-protecting groups such acetals, ketones, ketals, hydroxymethyl, tetrahydropyran-protected hydroxymethyl, dimethylcarbinol, or ethyl vinyl ether-protected carbinol. The removal of these groups often requires several steps and/or strongly alkaline media which tend to attack either the acetylenic linkage of the ethynyl group or any electron-withdrawing substituents on the aromatic nucleus. Therefore, there is a need for a simple process for preparing bis(ethynylphenyl) compounds in good yield that is suitable for large-scale synthesis operations. In particular, there is a need for a process for preparing 4,4'-diethynyldiphenylmethane and 2,2-bis(4-ethynylphenyl)hexafluoropropane in good yield.